In a continuously stratified fluid, the frequency of internal gravity waves depends only on the angle that the wavenumber makes with horizonal, i.e., the polarization relation in the non-rotational system. That is, the group velociity of internal wave is at right angles to the wavenumber vector, and hence its propagation characteristics are studied by "theory of ray-tracing". On the other hand, under the restriction on vertical scale, the behavior of internal wave can be frequently discussed by using "theory of decomposed mode-waves" or "two-layer approximate model". I try to do original interpretation as for the physical relationship between these three theories.